Laser debridement of wounds

ABSTRACT

A technique is disclosed for wound debridement in which a pulsed CO 2  laser beam is caused to impinge upon exposed tissue with individual pulses sufficiently energetic to ablate a thin layer of tissue. Each pulse has a time duration short enough to cause ablation but long enough so that it does not cause atmospheric breakdown. The CO 2  laser, with a wavelength in the far infrared region, is operated to produce a pulsed beam with individual pulses having an energy of about one joule per pulse or greater. The beam is focused to produce a beam diameter with a fluence of approximately ten joules per square centimeter at the tissue to be ablated and with a pulse repetition rate of approximately one hundred pulses per second and a pulse duration between one microsecond and ten microseconds. In operation, the ablation of eschar is achieved over a relatively wide range of angles of incidence without need for change of the beam parameters.

This is a continuation-in-part application of U.S. Ser. No. 07/679,421,filed Apr. 2, 1991 now U.S. Pat. No. 5,207,671.

FIELD OF THE INVENTION

This invention relates to the use of a laser as a medical instrument;more particularly, it relates to laser debridement of wounds.

BACKGROUND OF THE INVENTION

There has been a longstanding need for a new surgical technique fordebridement of wounds, especially burn wounds, which will significantlylessen the trauma and enhance healing. Laser surgery has held the hopefor fulfillment of the need but, for two decades, a successful lasertechnique has eluded the investigators.

Laser beams are used in a variety of surgical procedures. Earlierinvestigators have undertaken to utilize the laser for ablation oftissue in treating surface lesions of the skin and dermatologists areusing such devices today. However, there has been negligible success intreating burn wounds. While there are certain inherent advantagesafforded by laser surgery for serious burns, these cannot be availed ofunless the time required for the surgical procedure becomes at leastcomparable to that in the conventional scalpel surgery presently used inserious burn cases and unless the debridement leaves a tissue bed thatwill take a viable skin graft.

A serious burn on the human body usually extends well below thesuperficial skin surface. A significant thickness of tissue dies as aresult of the thermal assault and this necrotic material develops into ascar mass called eschar. The interface between the eschar and thesublying viable tissue is a fertile anaerobic region for growth andpropagation of infection. The current technique for medical treatmentinvolves debridement (i.e., removal) of the eschar by a surgicalprocedure called "tangential excision", accomplished with a scalpel.After the debridement, the exposed viable tissue is covered with a skingraft which provides protection against fluid loss and contamination andinitiates the healing process. This surgical procedure is known to behighly traumatic. Debridement by tangential excision for severe andextensive burn wounds may require many hours for the procedure and thereis an attendant very high loss of blood during the debridement. Mucheffort has been expended by investigators in seeking to develop a laserdebridement technique to replace the scalpel excision technique with itsattendant trauma.

The Prior Art

Levine et al have reported the investigation of the use of a carbondioxide (CO₂) laser for the excision of third degree burns with a viewtoward minimizing blood loss and minimizing damage to the underlyingtissue. See "Use Of A Carbon Dioxide Laser For The Debridement Of ThirdDegree Burns", Levine et al, Annals of Surgery, Volume 179, No. 2, pages246 through 252, February, 1974. This report describes an instrumentcomprising a CO₂ laser with an articulated surgical arm which permitsthe laser beam to be focused by a system of mirrors and an output lensin the handpiece held by the operating surgeon. The CO₂ laser isdescribed as having a power output which is variable from zero to fortywatts with a beam having a wavelength in the far infrared region at 10.6microns. It is reported that the laser was operated with a continuouswave output which enables use of the laser as a scalpel in generalsurgical procedures. The report describes an initial technique of "laservaporization" in which the beam was directly focused on the burnedtissue. It was reported that this method was found to be too slow andthat injury to underlying tissue often occurred. It also describes useof the laser as a scalpel, a technique referred to as "laser excision".In this, the laser is first used to incise a rim of tissue completelysurrounding the burned area; then a flap of tissue containing the burnedskin was dissected free of the underlying tissue, using the laser as a"photo knife" or scalpel. Laser debridement of burns using the same typeof CO₂ laser and similar procedure is described in "Laser Excision ofAcute Third Degree Burns Followed By Immediate Autograft Replacement: AnExperimental Study In The Pig" by Stellar et al, Journal of Trauma,Volume 13, No. 1, pages 45 through 53, January, 1973. In the describedprocedure with the CO₂ laser operated in the continuous wave mode, theremoval of tissue is described as "laser vaporization" in which thelaser is directly focused on the burned tissue, converting it to smoke.

Also, in the prior art, it is known to use the ablative effects of alaser beam in a dermatological procedure for removing thin lesions, suchas port wine scars and warts. In this technique, a CO₂ laser is operatedin a pulsed mode with the beam applied perpendicularly to the burnedsurface. This dermatological laser is operated with pulses of less than0.1 microseconds duration at a repetition rate of about one pulse persecond with two or three joules per pulse. This dermatological laser isinfeasible for removing eschar over large areas because it is too slow;it would, for example, take many hours to ablate the eschar from anentire limb.

Carbon dioxide lasers are currently available which, typically, emitpulses of infrared radiation at 10.6 microns wavelength in the regime ofseveral joules per pulse. These lasers have a characteristically shortpulse length, less than 0.1 microsecond, as an inherent feature of theirdesign. Such lasers are used for various purposes in the manufacturingindustry and are also used as surgical cutting instruments. Such lasersare of the so-called TEA type and are available from Lumonics Companyand Coherent Radiation, Inc., for example.

Events Leading To The Invention Disclosed In This Patent

We made the invention set forth below in the course of our undertakingto provide an improved technique for the debridement of burns which willgreatly reduce the time required for the procedure, impose significantlyless traumatic effect and enhance skin graft and healing of the wound.It was our objective to utilize a laser to ablate the eschar of a burnwound in such a manner that a high rate of removal would be realizedwithout significant injury to the viable tissue.

The above-mentioned dermatological CO₂ laser is not capable of achievingour objective because of the very low rate of ablation which can berealized. The use of a CO₂ laser of the TEA type was researched with theobjective of achieving a high rate of ablation by using a high pulserepetition rate, say 100 pulses per second. The TEA laser delivers anoutput energy of about three joules per pulse in a beam approximatelyone centimeter in diameter. The fluence of deposited energy with thisbeam, about four joules per square centimeter, is not enough to ablatethe surface layer of eschar. However, with an infrared transmittinglens, the diameter of the beam impinging on the surface can be reducedsufficiently so that the fluence, i.e. the joules per square centimeter,is high enough to ablate the surface layer. It was found that therequired fluence could not be achieved with the laser without causingatmospheric breakdown which was found to occur at a fluence of about tenjoules per square centimeter. The atmospheric breakdown and attendantelectric discharge commences within a small fraction of a microsecondafter laser pulse initiation and the resulting plasma shields the escharfrom absorbing the remaining energy from the laser pulse. Thus, thecommercial TEA type of CO₂ laser was found to be incapable of achievingour objective.

This led our investigation to the discovery that successful ablation canbe realized with a pulsed beam of correlated pulse duration and energyfluence. With such correlation, a high pulse repetition rate can beused. Further, it was realized that the laser beam energy is absorbed byonly a few wavelengths penetration of water-laden tissue and thus, thesurgeon could control the beam manually to ablate the eschar withoutsignificant injury to the viable tissue.

SUMMARY OF THE INVENTION

We have discovered that the objectives of our invention are achieved bya wound debridement technique in which a pulsed laser beam is caused toimpinge upon a layer of exposed tissue with individual pulsessufficiently energetic to ablate the thin layer of tissue, each pulsehaving a time duration short enough to cause the ablation but longenough so that it does not cause atmospheric breakdown.

Further, in accordance with this invention, the pulsed laser beam isproduced by a CO₂ laser having a wavelength in the far infrared region.Further, with a CO₂ laser, the individual pulses have an energy ofpreferably about one joule per pulse or greater. Further, the laser beamis produced with pulses having a duration between one microsecond andten microseconds. Further, the laser beam has a diameter at the tissueso that a fluence of approximately ten joules per square centimeter isrealized. Further, the laser beam has a pulse repetition rate in therange of approximately fifty to one hundred fifty pulses per second.

A complete understanding of this invention may be obtained from thedetailed description that follows.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of apparatus for carrying out theinvention; and

FIG. 2 depicts a laser beam with different angles of incidence on asurface.

BEST MODE FOR CARRYING OUT THE INVENTION

An illustrative embodiment of the invention will now be describedutilizing a pulsed CO₂ laser for debridement by ablation of the escharof a burn wound. It will be appreciated, as the description proceeds,that the invention may be realized in other embodiments and may beutilized in various applications.

Referring now to the drawing, apparatus is illustrated for carrying outthe invention. The apparatus comprises a CO₂ laser 10 and an opticaldelivery system 12 in the form of an articulated surgical arm. Thedelivery system 12 has its input coupled with the output of the laser 10and includes a beam output head 14 which is adapted to be hand-held andmanually manipulated with complete freedom of movement, both angularlyand translationally. The laser beam 16 is emitted from the end of thehead 14 and by manipulation thereof, the beam may be directed asdesired. As illustrated, the beam 16 is caused to impinge approximatelyperpendicularly on the surface of the eschar 18 of a burn wound. Thehead 14 (or a foot pedal) is provided with a control switch, not shown,for turning the beam 16 on and off. The surgical apparatus may alsoinclude lighting on the head 14, and a means for controlling the pulserepetition rate. Preferably, the apparatus is provided with means forclearing, from the path of the laser beam, smoke or other debrisresulting from the ablation of eschar or the tissue. It is alsodesirable to blow away any collected blood or other fluid in the area ofimpingement of the laser beam. For this purpose, the beam output head 14is fitted with an air conduit 24 which is provided with an output nozzle26 adjacent the laser beam and directed across the surface of the wound.An air blower, not shown, is connected with conduit 24. The nozzle isadapted to provide a thin, laminar stream of air flow over the surfaceof the wound at the area of beam impingement with a sufficiently highvelocity to clear the air and wound surface between laser pulses. For apulse rate of about one hundred pulses per second an air flow at thespeed of about mach 0.1 or 0.2 is desirable. In order to collect thesmoke and debris entrained in the air flow over the wound, a vacuumcleaner, not shown, is connected with a conduit 32 extending to anintake opening 34 opposite the nozzle 26. Thus, the beam path is clearedof smoke and other debris which would otherwise adversely affect thetransfer of energy from the laser beam to the wound.

The CO₂ laser 10 emits pulses of infrared radiation at 10.6 micronswavelength. The laser is capable of delivering individual pulses whichmay be selectively preset to comprise energy in the range of severaljoules per pulse. The laser may also be preset to produce pulses havinga duration between one and ten microseconds and to produce a pulserepetition rate in the range of approximately fifty to one hundred fiftypulses per second. The optical delivery system is provided with opticalfocusing to vary the beam diameter at the surface of the tissue so thata fluence of approximately ten joules per centimeter can be realized.The infrared energy is uniformly distributed over the cross-section ofthe beam within about ten percent variation. A laser with thesecapabilities is available from Plasmatronics of Albuquerque, N.M.

In accordance with the invention, the method of wound debridement is asfollows. The CO₂ laser 10 is operated to produce a pulsed beam withindividual pulses having an energy of about one joule per pulse orgreater. The beam is focused to produce a beam diameter at the tissue sothat a fluence of approximately ten joules per square centimeter isrealized. If the energy per pulse is increased, the beam size may beincreased to cover a larger area of tissue with the fluence beingmaintained at approximately ten joules per centimeter. The laser isoperated with a pulse repetition rate in the range of approximatelyfifty to one hundred fifty pulses per second with a pulse durationbetween one microsecond and ten microseconds. If the pulse duration ismuch less than one microsecond, atmospheric breakdown may occur andpreclude ablation of tissue. If the pulse duration is increasedsignificantly beyond ten microseconds, the result will be a verydeleterious smoking and cauterization, accompanied by heat penetrationinto the sublying living tissue.

With the laser operating as set forth above, the surgeon manipulates thebeam head 14 to cause the beam to impinge on selected eschar to ablatethe eschar over the required area. Experiments using this methoddemonstrate that eschar may be removed at a high rate withoutatmospheric breakdown. Experiments were conducted on a fullyanesthetized fifty pound piglet. Four burns, each of one square inch,treated with this method permitted remarkably successfully skin grafts.The surgeon was able to learn the technique in a matter of minutes eventhough the experimental optical delivery system was rather crude. It wasfound that the surgeon had no difficulty in controlling the beam inorder to achieve satisfactory ablation without significant damage tounderlying viable tissue. It is believed that the experimental workshows that the damaged layer is thin enough so that the attendantsubfusing of liquids is sufficient to adequately "wash" the bed and thusincrease the viability of the subsequent skin graft. The damage level isbelieved to be less than a few microns, an amount which has no clinicalsignificance to subsequent skin grafting.

In operation of the laser as described above, the beam is manipulated toablate successive regions of eschar. A high rate of ablation, asdiscussed above, is achieved when the beam impinges on a layer of escharat angles within a wide range of angularity from a directionperpendicular to the eschar layer. The ablation rate is substantiallyindependent of the angle of incidence within a given range. The reasonfor this is set forth below.

The effect of changing the angle of incidence of the laser beam isillustrated in FIG. 2. In this figure, the laser beam 16 emitted fromthe head 14 in one position (shown in uninterrupted lines) impinges uponthe surface 42 of a body 40. The beam 16 is perpendicular to the surface42 and has an angle of incidence equal to zero. With the laser head 14in an alternate position (shown in interrupted lines), the laser beam 16impinges upon the surface 42 with an arbitrary angle of incidence equalto A. The fluence of the beam in the area of impingement on the surface42 will decrease proportional to cosine A. This is because the areaimpacted by the beam will increase as 1/cosine A. This is the obliquityeffect. On the other hand, the depth to which the radiation will go(before intensity is reduced by about one-half) will decrease accordingto cosine A just because of penetration effects. The obliquity effect isillustrated by the difference of the impact areas shown in FIG. 2 as A1and A2 and the difference in depth of penetration is indicated by thedimensions D1 and D2 . Accordingly, the deposition of radiant energy perunit volume in the upper layer of tissue will be independent, to firstorder, of departures from perpendicularity over a wide range of angles.As the angle of incidence A is increased to values of about twentydegrees or more the energy per unit volume in the upper layer of eschar,and hence the ablation rate, may begin to decrease. At angles ofincidence greater than about thirty degrees, there is a diminution ofthe radiant energy per unit volume which may cause a decrease ofablation rate by a few percent, say about ten percent or less, of thatwhich is achieved with an angle of incidence equal to zero degrees.

The surface of the eschar has a topography which is characterized bysomewhat irregular contours due to anatomy as well as pits and ridges inthe eschar itself. The resultant angular variations relative to a beamdirected perpendicular to the surface are typically less than aboutthirty degrees. Accordingly, with the laser beam impinging upon a layerof eschar at angles ranging from about zero degrees to about thirtydegrees from a direction perpendicular to the eschar layer, high ratesof ablation are achieved. If the angle of incidence is increased muchbeyond thirty degrees, there may be some degradation of the ablationrate. This can be compensated for by changing the beam parameters toincrease the fluence of the beam. For this, the beam may be focused to asmaller diameter or the energy per pulse may be increased or acombination of both.

In some cases, depending upon the topography of the eschar, the rate ofablation may diminish undesirably upon movement of the beam from oneregion of eschar to another. This might arise, for example, where thebeam is moved from a relatively smooth region to a region having a veryrough surface, i.e. deep pits and high ridges. This may result in impactof the beam over an actual area considerably greater than thecross-sectional area of the beam so that there is a significant decreasein the effective fluence of the beam. In this case, the rate of ablationmay be restored by increasing the fluence of the beam. This can beaccomplished by optically focusing the beam to decrease the beamdiameter. Also, it may be accomplished by increasing the energy perpulse or by a combined adjustment of the beam diameter and energy perpulse.

In some situations, the topography may include a vertical ridge ofeschar or other unwanted tissue. In this case, the ridge may be mostefficiently removed by tilting the beam sufficiently to impact the ridgenear its base to excise it from the sub-lying tissue. For this purpose,it may be desirable to line-focus the beam, i.e. decrease the beamdiameter to a narrow line and concomitantly, if necessary, reduce theenergy per pulse so that a fluence of about ten joules per centimeter ismaintained.

As mentioned above, it is desirable to remove the smoke and other debriswhich results from ablation. Such debris tends to form a cloud over theablated area. By directing the laser beam at a large angle of incidence,the beam path can be more effectively cleared of debris and the splatteronto the laser and the beam transmission optics is reduced.

As used herein, the term "ablate" is used to mean the blowing off oftissues by the absorption of pulses of intense radiation such as thatproduced by a laser. In the ablation process with short pulsed lasers alarge quantity of heat energy is absorbed almost instantaneously in avery thin layer of the eschar which literally explodes. It is believedthat part of this is due to actual evaporation of surface tissue andpart of it arises from an enormous internal pressure developed by thevaporization of water in the tissue which, in turn, literally explodesthe material away from the surface. The term "instantaneous" as justused means time periods of one microsecond or so which are extremelyshort compared to the time it takes for heat to diffuse into sublyingtissue to the point where significant damage could be caused.

Further, as used herein, the term "atmospheric breakdown" is used tomean a phenomenon which occurs when the electric field component of abeam of radiant energy becomes strong enough to cause actual electricaldischarge characterized by a flash of visible light and accompanied by a"cap pistol sound". Atmospheric breakdown occurs when the intensity of a10.6 micron wavelength beam exceeds 20 to 50 megawatts per squarecentimeter, depending on the cleanliness of the air in the path of thebeam.

Although the description of this invention has been given with referenceto a particular embodiment, it is not to be construed in a limitingsense. Many variations and modifications will now occur to those skilledin the art. For a definition of the invention reference is made to theappended claims.

What is claimed is:
 1. The method of debridement of eschar from a burnwound comprising the steps of:producing a pulsed laser beam withindividual pulses sufficiently energetic to ablate a layer of eschar,said laser beam having an average power of at least one hundred joulesper second, each pulse having a time duration short enough to avoiddeleterious heat penetration and long enough so that it does not causebreakdown of the atmosphere, and causing said beam to impinge upon saidlayer of eschar at an angle of incidence ranging from zero degrees toabout thirty degrees, said angle of incidence being measured from adirection perpendicular to said eschar layer.
 2. The invention asdefined in claim 1 including the steps of:increasing the fluence of thebeam to increase the rate of ablation if the rate of ablation diminishesupon movement of the beam from one region of eschar to another becauseof a change of the topography of the eschar.
 3. The invention as definedin claim 2 wherein said step of increasing the fluenceincludes:decreasing the beam diameter.
 4. The invention as defined inclaim 2 wherein the step of increasing the fluence includes:increasingthe energy per pulse.
 5. The invention as defined in claim 1 whereinsaid step of producing a pulsed laser beam includes:producing said laserbeam with a CO₂ laser having a wavelength in the far infrared region at10.6 microns.
 6. The invention as defined in claim 5 wherein said stepof producing a pulsed laser beam includes:producing said laser beam withindividual pulses having an energy of about one joule per pulse orgreater.
 7. The invention as defined in claim 5 wherein said step ofproducing a pulsed laser beam includes:producing said laser beam withpulses having a duration between one microsecond and ten microseconds.8. The invention as defined in claim 5 wherein said step of producing apulsed laser beam includes:producing said laser beam with a beamcross-section at said eschar so that a fluence of approximately tenjoules per square centimeter is realized.
 9. The invention as defined inclaim 5 wherein said step of producing a pulsed laser beamincludes:producing said laser beam with the pulse repetition rate in therange of fifty to one hundred fifty pulses per second.